A study on an optical imaging apparatus has been promoted in medical fields. The optical imaging apparatus irradiates a living body with light from a light source such as a laser, and forms an image of information about the inside of the living body acquired on the basis of the incident light. The optical imaging technique includes photoacoustic tomography (PAT). The photoacoustic tomography irradiates a living body with pulsed light generated from a light source and detects an acoustic wave generated from a living tissue absorbing the energy of the pulsed light propagating through and diffused in the living body (Patent Literature 1). More specifically, the technique uses a difference in absorptance of light energy between a detection portion such as a tumor and the other tissues, and receives by receiving elements an elastic wave that is generated when the detection portion absorbs the light energy and instantly expands. By analyzing the detected signal, the information can be used in measurement for an optical characteristic distribution in a living body, and more particularly, quantitative measurement for a specific substance in the analyte, for example, glucose or hemoglobin contained in the blood. Hence, the information can be used for specifying the position of a malignant tumor accompanying with the growth of new blood vessels. In particular, a light-energy absorption-density distribution can be obtained.
With PAT, an acoustic wave that is generated as the result of absorption of light in a local detection portion of an analyte is measured, and hence local light absorption information can be acquired. A model is described below, in which an analyte is fixed to a flat plate, a light irradiation region with laser light is two-dimensionally set on a surface of the analyte, and the light irradiation region is sufficiently large for an imaging area. When d is a distance from a light irradiation point to the detection portion, an initial sound pressure P of the acoustic wave generated at the detection portion is expressed as follows:P(d)=Γμa(d)Φ(d)  Expression (1),where γ is a Gruneisen coefficient (heat-acoustic conversion efficiency), μa(d) is an absorption coefficient at the position of the distance d, and Φ(d) is a light intensity at the position of the distance d. The Gruneisen coefficient Γ, which is an elasticity value, is obtained by dividing a product of a thermal expansion coefficient beta and the square of the sound speed c, by a specific heat at constant pressure Cp. In general, the Gruneisen coefficient Γ is a substantially constant value. Hence, if a change in sound pressure P, which is the magnitude of the acoustic wave, is measured by time division, the product of μa and Φ, that is, the light-energy absorption-density distribution H can be obtained. Then, by dividing H by the local light intensity Φ(d), μa (d) can be obtained.
Also, it is assumed that Φ0 is a light quantity of pulsed light that is emitted on the surface of the analyte. Light is exponentially attenuated in the analyte by absorption and scattering as the light propagates away from the surface. That is, the local light intensity Φ(d) can be expressed as follows:Φ(d)=Φ0·exp(−μeff·d)  Expression (2),where μeff is an average equivalent attenuation coefficient of the analyte. From Expressions (2) and (1), Expression (3) is established as follows:P(d)=Γμa(d)Φ0·exp(−μeff·d)  Expression (3).